The invention relates to a process for producing degummed vegetable oils and gums of high phosphatidic acid content by removing non-hydratable phosphatides and iron from water degummed vegetable oils and the oils and the high phosphatidic acid gums obtained by this process. More particularly the invention relates to a process which yields an oil that can be physically refined and a gum having good emulsifying properties.
Crude vegetable oils as obtained by pressing and/or extracting oil seeds contain several compounds other than triglycerides. Some of these, such as diglycerides, tocopherols, sterols and sterol esters need not necessarily be removed during refining but other compounds such as phosphatides, free fatty acids, odours, colouring matter, waxes and metal compounds must be removed because they disadvantageously affect taste, smell, appearance and keepability of the refined oil.
Several unit operations exist for the removal of these unwanted compounds, the conventional water degumming-process being the first one. During this process water or steam (e.g. 3% water for soybean oil) is added to hot crude oil (e.g. 70.degree. C.) as a result of which a gum layer is formed (e.g. after a contact time of about 5 minutes) which is separated from the oil (e.g. by centrifuging) and processed into commercial lecithin. The resulting water degummed oil thus has a considerably lower phosphorus content than the crude oil but still contains phosphatides, the so-called non-hydratable phosphatides (NHP), the presence of which is considered to be undesirable in fully refined oil.
These NHP are commonly removed during alkali refining. This unit operation comprises the dispersion of an acid, e.g. phosphoric acid in water degummed oil (or crude oil), the addition of slight excess of caustic soda liquor and the separation of the soaps thus formed. The soapstock thus obtained contains the free fatty acids originally present in the crude or water degummed oil, some triglyceride oil and the NHP and other mucilaginous compounds such as sucrolipids and lipoproteins. This soapstock therefore has to be split prior to disposal both to recover fatty acids contained therein and to obtain a less polluting effluent. Nevertheless, because of the presence of organic residues resulting from triglyceride oils, NHP and other mucilaginous compounds this effluent can still pose disposal problems requiring an often costly solution.
The alkali refined, so-called neutral oil is then bleached by heating under reduced pressure with bleaching earth which is subsequently removed by filtration. Some triglyceride oil adheres to the bleaching earth and this constitutes a refining loss. For this reason as well as to minimize disposal problems of spent bleaching earth, its usage level is kept as low as possible.
Finally, volatile compounds are removed from the bleached oil by steam stripping under vacuum during the deodorisation process. If the main purpose of this unit operation is the removal of free fatty acids, it is commonly referred to as physical refining.
Physical refining has a number of advantages over alkali refining, the main advantage being the avoidance of soapstock formation. A second advantage is the potentially lower refining loss because it avoids the saponification of oil and oil entrainment by the soaps as encountered during alkali refining. If, on the other hand more bleaching earth has to be used prior to physical refining than is required prior to deodorisation, this advantage may be more than offset.
Accordingly, physical refining tends to have economic advantages over alkali refining for oils with a high free fatty acid content such as palm oil, but there is another reason why oils such as soy bean oil, sunflower seed oil etc. are not commonly physically refined: the oils to be physically refined must be free from NHP in order to yield stable fully refined oils and the water degumming process does not remove NHP.
Consequently, a number of processes have been described that provide a clean, NHP-free feedstock for physical refining. In Dutch patent application No. 78 04829, a process is described that is concerned with physical refining of soy bean oil. It requires that the flaked soy beans be wetted and heated prior to being extracted. Oil extracted from such flakes shows a very low NHP-content after water degumming and is amenable to physical refining and thus yields a stable oil. Oil yield on extraction is, however, somewhat decreased, energy requirement during extraction is increased and although lecithin yield is considerably increased, the lecithin composition is changed (M. Kock, Fette, Seifen und Anstrichmittel 83, 552 (1981), Table 8).
Another process is described in DE-AS No. 26 09 705. In this process, water degummed oil is treated with an acid and cooled to below 40.degree. C. whereupon the NHP's form gums in a form that can be removed. In the specification it is noted that less acid is required if a crude oil is used instead of a water degummed oil, which discovery has led to another process as described in East German Patent No. 132 877 in which process lecithin is added to water degummed oil to facilitate the NHP removal.
This same discovery also forms the basis of British Patent No. 1 565 569 where a single separation degumming process for triglyceride oils is described, as part of the crushing operation. In this process an acid is added to a crude oil and allowed to contact the oil for a period of approximately 10 minutes for reaction whereupon this acid is at least partially neutralized by a base, an extended contact time being allowed for the development of a gum layer which is then separated without the need to cool. The gums thus obtained are not commercialized as such but passed to the meal desolventiser in a solvent extraction plant or added to the meal being pelleted.
As mentioned before crude vegetable oils besides other undesirable components contain metal compounds, the most usual metals being calcium, potassium, magnesium, aluminum, iron and copper. These metal impurities form salts of phosphatidic acid in the non-hydratable phosphatides, NHP. Further the metals are present as soap and are bound to other accompanying lipids. Metal contaminants and especially iron may cause darkening of the oil during deodorisation and even small amounts of iron which do not infringe the colour of the oil severely reduce the stability of the finished oil. Thus besides the removal of non-hydratable phosphatides also the removal of metal contaminants and especially iron is highly desirable in an economical degumming process. However, known processes usually only lead to a quite unsatisfactory reduction of the metal contents and especially the iron contents of the degummed oil.